This invention relates to a velocity measuring device and is concerned with a transducer for measuring the relative velocity of movement of a fluid medium, for example water or air, either when the transducer is made to move through the fluid medium or the fluid medium moves towards the stationary transducer.
Many velocity transducers are in common use but most require reciprocating parts that wear or become stuck for one reason or another. For example, when used in a marine environment, the paddle wheel type must be serviced regularly because it is affected by weeds, barnacles and mud. For air or gas velocity measurement, the bearings of the rotating part must be of sufficient high quality to provide a reasonably long life and hence long-term, reliability.
It is an object of this invention to provide a velocity transducer that does not rely on reciprocating parts or bearings and will not require regular servicing even in a marine environment.
According to the invention, there is provided a fluid velocity transducer comprising a substantially tubular body having a fluid-tight outlet for signal wires, the body having located therein a Hall effect unit, and a pair of oppositely facing diaphragms positioned respectively at each end portion of the tubular body, each diaphragm having a magnet fixed to its interior surface, the arrangement being such that in use one of the diaphragms can receive an applied force due to a fluid and convert this force into a displacement relative to the Hall effect unit thereby increasing the magnetic field strength applied to the Hall effect unit so as to produce a signal output which can be processed to provide an indication of the speed of movement.
The transducer of the invention measures velocity by making use of a very small displacement produced by the force of the medium in which it moves or the force of the medium that moves on to it. The transducer responds to the differential forces applied to its surfaces and produces a signal output only when the two forces applied are unequal. The output signal is proportional to the velocity in the direction of motion and very little or no output is produced in other directions, except in the reverse direction if the transducer or medium is reversed.
The tubular body of the transducer may be made from any material suitable for the environment in which it is to be used. For a marine application, a plastics material such as, for example an ABS (acrylonitrile-butadiene-styrene) copolymer, is suitable for fabrication of the tubular body and also of the diaphragms. The Hall effect unit is advantageously mounted on a printed circuit board positioned in a central region of the tubular body. The diaphragms can be fixedly attached to the respective ends of the tubular body, for example by chemical bonding or thermal welding, so as to provide a fluid-tight seal for each end of the tubular body. The magnets attached to each diaphragm respectively are preferably of different sizes.
The velocity transducer of the invention can be used to measure the speed of movement of a vessel, for example a ship or boat, through water, for which purpose it will be fixed to the hull of the vessel below the waterline. Other uses may be to measure the speed of water flow in a river or the flow rate of a liquid or gas through a pipeline. It may also be used to measure wind speed on land or the sea.
The signal output from the Hall effect unit may be coupled to an interface unit to process the varying direct current signal from the transducer for digital display.
If desired, a thermistor may be attached to the printed circuit board in addition to the Hall effect unit to enable the ambient temperature of the liquid or gas to be measured.